This invention relates to the field of process control. More particularly, the invention relates to monitoring the height of a liquid in a container as the fluid is used during a process step of manufacturing an integrated circuit.
In processing integrated circuits, such as semiconductor devices, titanium nitride is often deposited by metal organic chemical vapor deposition (MOCVD) to achieve highly conformal step coverage, such as in high aspect ratio vias and contacts. Typically, the deposition process employs tetrakis-dimethylamino titanium (TDMAT) as the precursor chemical. To transfer TDMAT into the deposition chamber, a carrier gas, such as helium, is bubbled through liquid phase TDMAT that is provided in a heated ampoule. The carrier gas conveys the TDMAT in the vapor phase to the deposition chamber.
During processing, care is typically used to ensure that the TDMAT ampoule does not run empty or below a minimum threshold amount, such as a minimum height level within the ampoule. If the TDMAT liquid in the ampoule drops below the minimum threshold amount, the desired thickness of titanium nitride may not be deposited during the MOCVD process, typically resulting in a significant reduction in process yield.
Several approaches to monitoring TDMAT levels have been attempted, including (1) estimating the TDMAT usage based on a processed substrate count, (2) using a pressurizing test to determine the empty volume in the ampoule, (3) using discrete level sensors in the ampoule, which sense an object floating on the liquid and detect the location of the floating object as it passes by discrete points, and (4) using sonic level sensors in the ampoule. However, each of these prior methods have significant disadvantages which limit their effectiveness and applicability. For example, the first method provides an inaccurate estimation, the second method cannot be accomplished in real-time and is too time-consuming and obtrusive, and the third and fourth methods are cost-prohibitive.
What is needed, therefore, is a system for real-time, continuous, unobtrusive, and inexpensive monitoring of a process liquid level in an ampoule.
The above and other needs are met by a system for indicating an amount of a process liquid contained within an interior of an ampoule. A first conduit is in selective fluid communication with the interior of the ampoule, and has a first opening configured for disposal below an upper surface of the process liquid. The first conduit introduces a carrier gas into the interior of the ampoule. A second conduit is also in selective fluid communication with the interior of the ampoule, and has a second opening configured for disposal above the upper surface of the process liquid. The second conduit receives the carrier gas from the interior of the ampoule.
A pressure differential sensor is disposed between and is in selective fluid communication with the first conduit and the second conduit. The pressure differential sensor senses a pressure differential between the first conduit and the second conduit. An indicator indicates the amount of the process liquid in the ampoule, based at least in part upon the pressure differential between the first conduit and the second conduit.
Thus, the system as described above provides selectively continuous monitoring of the amount of the process liquid that is in the ampoule. Furthermore, the system monitors the amount in real time, and is relatively inexpensive to implement in a new chemical vapor deposition system, or to add to an existing chemical vapor deposition system.
In various preferred embodiments of the invention, the amount of the process liquid in the ampoule is expressed as a height of the process liquid, such as a height from the bottom of the ampoule to the upper surface of the process liquid, or a height from the first opening of the first conduit to the upper surface of the process liquid. The pressure differential sensor is preferably a manometer, and most preferably a U tube manometer containing manometric fluid, and having a first arm in fluid communication with the first conduit and a second arm in fluid communication with the second conduit.
In one preferred embodiment, graduated indicia on at least one of the first arm and the second arm of the U tube manometer indicate a difference in height of the manometric fluid in the first arm and the second arm. A conversion table indicates a height of the process liquid in the ampoule based at least in part upon the difference in height of manometric fluid in the first arm and the second arm. Alternately, the graduated indicia on at least one of the first arm and the second arm of the U tube manometer directly indicate a height of the process liquid in the ampoule, based at least in part upon the difference in height of manometric fluid in the first arm and the second arm.
In an especially preferred embodiment, the indication of the amount of the process liquid in the ampoule is based at least in part on:       h    =                            Δ          ⁢                      xe2x80x83                    ⁢          P                -        C                    ρ        xc3x97        g              ,
where
h is a height of the process liquid in the ampoule,
xcex94P is the pressure differential between the first conduit and the second conduit,
C is a constant based at least in part on a configuration of the ampoule,
xcfx81 is a density of the process liquid, and
g is an acceleration due to gravity.
According to other aspects of the invention there is provided a chemical vapor deposition system including the system for indicating an amount of a process liquid as described above, and a method for determining an amount of a process liquid in an ampoule.